I believe ETF's are the greatest advancement in personal investing of the past three decades.  Nevertheless, they are not without peril.  The attached article points out how hedge funds are currently using some of the more esoteric ETF's for some of their trading strategies.  The article also points out how the built-in tracking error (usually quite small) can increase greatly during periods of high volatility in thinly traded markets.  All this being said, I still believe that ETF's, used properly, offer the retired investor a considerable advantage of mutual funds.